Radiotherapeutic apparatus involves the production of a beam of ionising radiation, usually x-rays or a beam of electrons or other sub-atomic particles. This is directed towards a cancerous region of the patient, and adversely affects the tumour cells causing an alleviation of the patient's symptoms. Generally, it is preferred to delimit the radiation beam so that the dose is maximised in the tumour cells and minimised in healthy cells of the patient, as this improves the efficiency of treatment and reduces the side effects suffered by a patient. A variety of methods of doing so have evolved.
One principal component in delimiting the radiation dose is the so-called “multi-leaf collimator” (MLC). This is a collimator which consists of a large number of elongate thin leaves arranged side to side in an array. Each leaf is moveable longitudinally so that its tip can be extended into or withdrawn from the radiation field. The array of leaf tips can thus be positioned so as to define a variable edge to the collimator. All the leaves can be withdrawn to open the radiation field, or all the leaves can be extended so as to close it down. Alternatively, some leaves can be withdrawn and some extended so as to define any desired shape, within operational limits. A multi-leaf collimator usually consists of two banks of such arrays, each bank projecting into the radiation field from opposite sides of the collimator.
The leaves on the MLC leaf bank need to be driven in some way. Typically, this is by a series of lead screws connected to geared electric motors. The leaves are fitted with a small captive nut in which the lead screws fit, and the electric motors are fixed on a mounting plate directly behind the leaves. Rotation of the leadscrew by the motor therefore creates a linear movement of the leaf. The leaf drive motors are inevitably wider than a single leaf thickness, so in order to be able to drive each leaf the motors have to be mounted in a particular pattern as shown in FIG. 1. This shows a housing 10 for an array of adjacent MLC leaves 12. Behind the array, a motor mount 14 is fixed in place to housing 10 via bolts 16 so that it lies behind the leaves 12. A motor 18 for each leaf 12 is fixed to the motor mount 14.
Each motor 18 is generally tubular and from one end (as shown in FIG. 1) therefore appears circular. The motors are wider than an individual leaf and are therefore arranged in a staggered pattern. In this example, the motors 18 are arranged in four offset rows so that the centre of a motor is aligned with each leaf. As a result of this, the leadscrew nuts therefore have to be fixed to the leaves in one of a variety of positions, meaning that (in this case) four different leaf shapes need to be manufactured.
In an alternative system referred to as the “Beam Modulator” and shown generally in FIG. 2, leaves are driven by a rack and pinion system. A gear rack 20 is machined into the top or bottom of the leaves 22 and is driven by motors 24 fixed to the side of the leaf bank. The motor gear pinions 26 are mounted to an extension shaft 28 of a suitable length to enable the drive to be carried across to the appropriate leaf to be actuated.
In our earlier patent application GB-A-2423909, we describe a modular design similar to the Beam Modulator drive system. The application describes a design where a system of miniature gears and racks are incorporated into a detachable module. The linear motion is transmitted to the leaf via a slotted feature in the rack and engages in a leaf drive coupling fitted to the rear of the leaf.
The choice of drive system is influenced by the quantity and thickness of the leaves in the leaf bank. For example, the MLC leaf bank has 40 leaves per side and has an average leaf thickness of 3.6 mm. This thickness and number of leaves allows for a conventional solution of placing the motors directly behind the leaves and actuating them via a leadscrew which passes through the centre of the leaf.
The diameter of the leadscrew in this design is limited to 2.5 mm, as this is largest diameter that can pass into the leaf without interfering with neighbouring leaves. Conveniently, it is also a standard ISO thread size. The leadscrew has to drive a leaf weighing around 800 g, and at certain head/gantry angles the full weight of the leaf is suspended by the thread alone. Due to the small engagement area of the thread, the leadscrew therefore experiences high frictional loads and requires regular lubrication to maintain an acceptable service life. The performance of the leadscrew is also adversely affected by a whipping motion that can arise when the leaf nut is close to the motor, in which the long free end of the leadscrew can oscillate as it rotates. In addition, the leadscrew experiences a buckling load when the leaf is pushed to the far end of the leadscrew. There is also a certain degree of noise due to this motion of the leadscrew.
The Beam Modulator design employs a thinner leaf in order to increase the resolution of the leafbank. This leaf thickness of only 1.75 mm influences the selection of the drive system. A lead screw system as used on the MLC would not be a viable solution as it would require a 1.5 mm diameter leadscrew; as the leaf travel is longer, the leadscrew would suffer increased whipping and buckling. Leadscrews with a high aspect ratio are also extremely difficult and costly to manufacture and are likely to fracture if they are not adequately supported. In addition, the number of motors required (40 per side) could not be fitted in behind the leaves due to their size.
The drive system for Beam Modulator therefore incorporates a rack and pinion system, with the motors disposed on either side, top and bottom of the leaf bank. The motors are fixed to the side of the leaf bank, and pinions are driven from the motors on extension shafts requiring 10 different lengths, in addition a staggered bearing block is incorporated in which the extension shafts runs. 8 such bearing blocks are required for the leaf bank.
Because the motors are dispersed along the 4 sides of the leaf bank, the bank has to be removed for motor servicing. Removal of the leaf bank is a lengthy process, and problems can occur with radiation performance if the leaf bank is not replaced in the same position.
The rack is machined into the top or bottom of the tungsten leaf; the bearing surface that would be positioned at the top of the leaf therefore has to be offset in order to make way for the rack. This has the undesired effect of reducing the shielding effect of the leaf, as some 8 mm is lost off the top/bottom of the leaf for the rack and bearing surface.
In order for smooth operation of the rack a certain amount of clearance has to be maintained between the rack and pinion. Each of the 80 motors therefore has to be checked when assembling the leaf bank. This clearance can vary leaf to leaf, depending on manufacturing tolerances, and can lead to unwanted backlash once the pinion and motor gearbox begin to wear. Such backlash will affect the positional accuracy of the leaves.
GB-A-2423909 describes a removable module which alleviates many of the service issues problems experienced with the beam modulator. However, as it incorporates a rack and pinion system it will suffer from backlash in the same way. The MLC Rack and Pinion System was originally designed around a 160 leaf MLC, but limitations in available space in the treatment head above and below the leafbank as well as restrictions on the overall head diameter create problems for fitting this type of Actuator. The gear racks in the actuator are positioned to match the leaf pitch; during operation the racks extend into the radiation beam, which may have effects on beam performance—particularly if there is an error in the pitching. The Actuator module also contains a high part count, including many precision cut gears and racks making this expensive to produce.
Thus, the leaf thickness/pitch and motor size affects the method in which the actuation is carried to the leaf, and once a suitable method is derived (of the 2 practical drive solutions, leadscrew and rack and pinion) the design can have inherent problems with wear, noise, production and assembly costs, backlash and servicing issues.